In 31 young men and women, cardiac MRI revealed that contractility of the left ventricle was significantly increased an hour after consuming an energy drink, according to Jonas Doerner, MD, of University Hospital Bonn in Germany, and colleagues.

Most of the effect on ventricular contractility appeared to be due to the taurine contained in the drinks, rather than the caffeine. This naturally occurring amino acid in the body has been shown to increase release of calcium in muscles, Doerner said.

But it's not known whether these changes have any actual clinical impact, he warned. That's something that needs to be looked at in larger studies.

David Hovsepian, MD, of Stanford University, who wasn't involved in the study, questioned whether changes in ventricular contractility add up over time.

"Does it correlate to a harmful effect, and is it something that is temporary while these substances are in your body, or is there a lingering effect?" Hovsepian told MedPage Today. "There are people who consume these drinks on a regular basis. It would be interesting to see."

Doerner said he and his team wanted to examine the effects of energy drinks on cardiovascular function because the market for these beverages is booming and has increasingly been associated with adverse outcomes.

Sales for the multibillion dollar industry are growing every year, he said, and now there are more than 500 different brands available around the world. These energy boosters are full of compounds that have known stimulant effects, including caffeine and taurine.

Most of them actually have three times the caffeine of normal beverages, he added. While the FDA says caffeinated sodas can't have more than 20 mg/100 mL of the substance, energy drinks usually pack 32 mg of caffeine into every 100 mL.

And additives, like taurine, may compound the stimulant effects of caffeine. That's why it's not surprising that emergency department visits related to energy drinks doubled from 2007 to 2011, according to a recent report from the Substance Abuse and Mental Health Services Agency (SAMHSA). In the 2011, about 20,000 ED visits nationally were linked to these buzz-inducing beverages.

To assess the effects on the heart, Doerner and colleagues conducted cardiac MRI on 31 healthy patients whose mean age was about 28. They were all scanned before and one hour after having an energy drink that contained 32 mg/100 mL of caffeine and 400 mg/100 mL of taurine.

After an hour, there were no changes in heart rate, blood pressure, or LVEF, and there were only slight significant increases in left ventricular end diastolic volume and left ventricular stroke volume.

But there were significant increases in peak systolic strain (a 7% increase) and in peak systolic strain rate (about a 6% increase).

Doerner explained that strain is produced by the application of stress and is a measure of the heart's ability to contract. It isn't clear, however, what this means clinically.

To try to assess which component of the energy drinks was responsible for the changes, the researchers looked again at 10 of the patients for changes in the heart brought on solely by caffeine.

But caffeine alone didn't produce the same effects: particularly, there was no impact on ventricular strain.

The stimulant led to a nonsignificant increase in heart rate and a significant rise in diastolic blood pressure, but little impact on LVF, with the exception of a small but significant decrease in left ventricular end diastolic volume.

This indicates that the effects of the energy drink may be due to the taurine, or potentially a synergistic effect of caffeine and taurine.

Doerner said the long-term cardiovascular effect of these beverages on healthy individuals warrants further assessment, as does their effect on those with established disease.

Doerner reported no conflicts of interest. A co-author reported relationships with Medtronic.

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